Role of hydrogen in Nd-Fe-B sintered magnets with DyHx addition

In order to improve the coercivity of Nd-Fe-B sintered magnets, DyH2 and DyH3 fine powders were prepared and used as additive for preparing Nd-Fe-B sintered magnets. The effects of DyHx powders addition on the microstructures and the magnetic properties of the magnets have been investigated. It was found that hydrogen will react with oxygen of NdOx rich intergranular phases to form Nd rich phases by dysprosium hydride addition. The Nd-rich grain boundary phases are more homogenous and continuous because the volume fraction of Nd-rich grain boundary phases increases with respect to the Nd oxide phases. After desorption, fine dysprosium powders become more active and wrap matrix phases well so that the diffusion of dysprosium to the surface layer of matrix phases is convenient, so dysprosium decreases in grain boundary phases and aggregates in surface layer of matrix phases. Then, intrinsic coercivity of NdFeB sintered magnets is improved from 14.96 kOe to 20.5 kOe and 20.31 kOe by 2.0 wt.% DyH3 and 2.0 wt.% DyH2 addition, respectively. This study has shown that DyHx addition can reduce the content of oxygen in grain boundary phases. This can be an effective method for massive production.